1. Field of the Invention
The present invention relates to an image reading apparatus for reading an original document with a linear image sensor such as a charge-coupled device.
2. Related Background Art
In conventional facsimile apparatus, intermittent subscanning operation in reading an original document is achieved by moving the document or a linear image sensor with a stepping motor.
Such facsimile apparatus is provided with functions such as varying the image reading density or reducing the original image. More specifically, in a structure in which the subscanning operation is conducted by moving the linear image sensor or the original document by a stepping motor in a direction substantially perpendicular to the main scanning direction, or perpendicular to the direction of array of the linear image sensor, a variation in the image magnification or in the image density in the subscanning direction is achieved by controlling the pulses supplied to said stepping motor.
In this manner the variation in image density or in image size has been achieved by changing the frequency of driving pulses for the stepping motor or by mechanically changing the subscanning speed. However, such a method does not allow one to obtain an arbitrary image size or density, because of limitations on the selectable pulse frequency or on the mechanism.
FIG. 1 illustrates an example of an original reading apparatus utilizing a linear image sensor.
A contact-type linear image sensor 1, having a plurality of photosensor elements arranged over the original reading width, and an exposure light source 2 constitute an optical system 3, which is driven by a driving system consisting of a stepping motor 5, a driving belt 6, a pulley 7 and another pulley 8, in a subscanning direction perpendicular to a main scanning direction, equal to the direction in which the array of the photosensor elements extends, thereby reading an original document placed, with the image-bearing face thereof downwards, on an original support glass plate 4.
In the continuous movement of the optical system through continuous rotation of the stepping motor 5, the frequency of driving pulses therefor, or the scanning speed, is selected to be lower in order to achieve an enlarged or high-density image reading. On the other hand, the frequency of said driving pulses or the scanning speed is selected to be higher in order to achieve a reduced or low-density image reading.
However, in applications where intermittent feeding is required, such as the facsimile apparatus, a simple change in the frequency is unable to provide a satisfactory change in the image size or density, because of possible effects on the obtained image, unless the intermittent movement of the optical system is synchronized with the cycles of charge accumulating operations of the linear image sensor.
This is shown in more detail in FIG. 2.
In FIG. 2, it is assumed that the contact-type linear image sensor 1 carries out accumulation of photoinduced charges at a constant pitch as shown by (1)-(4), and accumulation of photoinduced charges during the movement of the optical system is effective. FIG. 2 shows a case where the optical system is stopped during an accumulating period (3), but the duration of charge accumulation does not coincide with the start and stop of the optical system as indicated by (a) and (b) since the phases of the driving pulses are not synchronized with the charge accumulating operation of the contact-type linear image sensor 1.
The image signal obtained in the accumulating period (3) is discarded, so that the image data obtained in the period (a) are lost and the image quality is therefore reduced.
Also in the accumulating period (4), the amount of movement is decreased by the period (b) so that the image quality is again reduced.
In this manner the phases of the driving pulses for the stepping motor 5 have to be synchronized with the charge accumulating operations of the contact-type linear image sensor 1 in order to obtain a satisfactory image quality.
However, for example in an optical system in which three pulses are given to the stepping motor 5 during an accumulating operation of the linear image sensor 1 for reading the image in actual size, an enlarged image reading of 150% requires a movement of the optical system corresponding to 1.5 pulses during an accumulating operation, so that the phases of the driving pulses for the stepping motor cannot be matched with the charge accumulating operations of the image sensor in a case of intermittent movement. For this reason it has been difficult to obtain an arbitrary image magnification or image density in the use of a stepping motor.